Cannabis can precipitate the first episode of psychosis
If someone has a predisposition to a psychotic illness, such as schizophrenia, use of drugs such as cannabis may trigger the first episode in what can be a lifelong, disabling condition. There is increasing evidence that regular cannabis use precedes and causes higher rates of psychotic illness. Psychotic illnesses are characterised by:

* Delusions - for example the person believes they have special powers.
* Hallucinations - for example the person hears voices or sees things that aren't really there.
* Thought disorder - for example the person has difficulty organising their thoughts.

When people experience psychotic symptoms, they are unable to distinguish what is real. They lose contact with reality.
http://www.betterhealth.vic.gov.au/b...psychosis?open

Results On an individual level, cannabis use confers an overall twofold increase in the relative risk for later schizophrenia. At the population level, elimination of cannabis use would reduce the incidence of schizophrenia by approximately 8%, assuming a causal relationship. Cannabis use appears to be neither a sufficient nor a necessary cause for psychosis. It is a component cause, part of a complex constellation of factors leading to psychosis.

Conclusions Cases of psychotic disorder could be prevented by discouraging cannabis use among vulnerable youths. Research is needed to understand the mechanisms by which cannabis causes psychosis.
http://bjp.rcpsych.org/cgi/content/abstract/184/2/110

It should be noted from the outset that:

* Much of the evidence for cannabis-associated psychosis has been based on subjects who have used large doses of cannabis and/or used cannabis chronically

* A great deal of the data has been derived from small studies, some of which have contained inherent flaws

and

* The findings of some of the larger studies conducted over the last 20 years have varied considerably
----
It is believed that cannabis use may cause a condition known as a drug-induced psychosis or cannabis psychosis which:

* Can last up to a few days

* Is often characterised by hallucinations, delusions, memory loss and confusion

* Usually results from prolonged or heavy cannabis use

and

* Responds well to treatment

However, in some cases, cannabis use may contribute to the development of a psychosis such as schizophrenia which:

* Constitutes a serious mental illness for the majority of people with the disorder

* Is characterised by hallucinations, delusions, social withdrawal, paranoia, self-neglect, thought withdrawal, and speech disorders like associative or incoherent speech

and

* Tends to respond less well to treatment than cannabis psychosis

Most of the research in this area has investigated cannabis in people with schizophrenia.

Does cannabis use cause psychosis?
* There are a number of risk factors which are believed to contribute to the development of a psychosis, one of which is believed to be cannabis use

* The peak age range during which people are more vulnerable to developing a psychosis (16-30 years of age) tends to be the same age range during which people tend to use cannabis

* A very large study of 50,465 Swedish army conscripts determined their cannabis use at age 18 and followed those individuals for the next 15 years. Recruits who had tried cannabis by age 18 had 2.4 times the risk of being diagnosed with schizophrenia in the following 15 years than those who had never used cannabis. The risk increased if the conscript used greater quantities of cannabis

However, of the conscripts who developed schizophrenia:

* Most had never used cannabis

and

* Only 7.7 per cent were heavy cannabis users

* Therefore, cannabis use is only one possible factor contributing to an increased risk of developing schizophrenia or other psychosis. This is especially true when one considers that most people who develop a psychosis have never used cannabis

* In addition, cannabis use at age 18 might be a consequence of emerging psychosis (that is, psychosis precedes cannabis use, not vice versa)
http://www.health.vic.gov.au/drugser...s/cannabis.htm

Cannabis: Too much, too young?

* 26 March 2005
* Graham Lawton
* Magazine issue 2492

It is known that taking a lot of cannabis over a short period can cause psychotic symptoms, but the long term effects are still hotly debated

http://www.newscientist.com/channel/...mg18524921.300
(Don't waste your time on that link.)

Who is most at risk from cannabis use?

* People most at risk are those with a family history of psychotic illness or those who have already experienced a psychotic episode

* Thus, people with a family or a personal history of psychotic illness should avoid drugs like cannabis completely or at least attempt to minimise their use
http://www.health.vic.gov.au/drugser...s/cannabis.htm
(same as above)